CLASSIFICATION OF ANIMALS. 11 



the anatomist, in the next place, has sought to discover the difference 

 which, added to the definition of animal^ would form the most ex- 

 tended species of that genus. 



Ai'istotle thought he had found this primary differential character 

 in the blood, recognising as blood only the circulating fluid which 

 was red coloured. He accordingly divided the Zoa into Enaima 

 and Anaima, or the sanguineous and exsanguineous animals. For 

 a long time no advance was made beyond this early step in the 

 primary division of animals. It was at length discovered that many 

 of the exsanguineous animals of Aristotle did actually possess blood, 

 though differing in colour from that of the so-called sanguineous 

 species. This led, however, only to a nominal improvement in the 

 classification ; the Enaima were called " red-blooded," the Anaima 

 "white-blooded" animals. It was reserved for Cuvier* to discover? 

 in the course of his minute dissections of the lower animals, that an 

 extensive class of worms had red blood circulating in a closed system 

 of arteries and veins ; and this discovery first materially affected the 

 value of the character applied by Aristotle to the primary groups of 

 the animal kingdom. 



The Annelides, or red-blooded worms, could not, however, be com- 

 bined with birds, beasts, and fishes, in a natural system, since they 

 differed from them widely in almost every other particular of their 

 organisation. 



Some other character was therefore sought for, since it became 

 obvious that the colour of the blood led to an artificial combination of 

 species. Lamarck | thought he had discovered the desired character 

 in the vertebral column, this structure being present in all the Enaima 

 of Aristotle, and absent in all his Anaima. Lamarck proposed, 

 therefore, the name of Vertebrata for the one class, and Invertebrata 

 for the other. Now it will be observed that the Invertebrata are 

 grouped together by a negative character ; and I know not any in- 

 stance where such a character has been employed in zoology, in which 

 very differently organised species have not been associated together. 

 What indeed can be predicated in common of the snail, the bee, and 

 the polype, than that they are animals, have no red blood, no vertebral 

 column, and the like negations ? It was obvious also that the Verte- 

 brata and the Inverte])rata were not coequal groups, and the idea of 

 equivalency or proportion, as well as that of likeness, ought always 

 to govern the labours of the classifier. 



In the attempt to remedy this defect, the discovery was made that 

 the vertebral column was subordinately related to a condition of a 

 much more important system in the animal body than the skeleton, 



* XII. t. ii. p. 515. t Xin. 



